The present invention relates to a tuning frequency automatic control device of a receiver, and more particularly to a voltage synthesizer type tuning frequency automatic control device of a receiver which is capable of automatically selecting broadcasting stations.
In a receiver of a voltage synthesizer type, as shown in FIG. 1, after being amplified by an RF amplifier 1, an RF (radio frequency) reception signal is subjected to frequency conversion by a mixer 2 to provide an IF (intermediate frequency) signal which is in turn provided through an IF amplifier 3 to an FM detector 4. The FM detector 4 is provided with a frequency discriminating function and it produces an output having an S-curve characteristic relative to the frequency variation. The S-curve output voltage is applied to a window comparator 5 to provide detection signals S.sub.1 and S.sub.2 when the S-curve output voltage is out of a particular range. These detection signals S.sub.1 and S.sub.2 are used to indicate detuning conditions.
A controller 6 implements a desired operation in response to the detuning detection signals S.sub.1 and S.sub.2 as well as up-instruction and down-instruction signals. The controller 6 produces a control voltage to be applied to a VCO (voltage controlled oscillator) 7. The output of the VCO 7 is used as a local oscillation signal to be applied to the mixer 2, thereby automatically controlling the tuning frequency.
FIG. 2 is a waveform diagram for description of one operation mode of the voltage synthesizer type receiver as shown in FIG. 1, which has been proposed by the applicant. When the up-instruction signal is applied from an external source, clock pulses of a predetermined frequency are up-counted by an up/down counter contained in the controller 6. In accordance with the count value in the counter, a DC voltage is generated by a combination of a D/A converter and a LPF (lowpass filter) and the DC voltage thus generated serves to vary the oscillation frequency of VCO 7, thereby increasing the tuning frequency.
When the S-curve output level falls below a lower threshold value V.sub.D of the window comparator 5 at time t.sub.1, the detuning detection signal S.sub.1 is produced from the comparator 5. In response to the signal S.sub.1, the clock pulses are subjected to frequency division, for example 1/8 frequency division, so as to lower the searching speed, and the clock pulses thus frequency divided are fed to the up/down counter so as to effect up counting. The reason for lowering the searching speed is to cover the delay caused by the time constant of the LPF which produces the DC voltage output of the control circuit. When the frequency enters into a tuning range at time t.sub.2, the detection signal S.sub.1 goes down to L level and the window comparator 5 no longer produces the signal S.sub.1. From this time instant, the frequency is further increased while maintaining the searching speed unchanged. Another detuning signal S.sub.2 is produced at time t.sub.3 when the S-curve output level reaches the upper threshold value V.sub.U of the window comparator 5. In order to measure the time period from t.sub.2 to t.sub.3, another up/down counter is employed which effects up-counting of one-half of the clock pulses (4) generated in this time period. In response to the detection signal S.sub.2, the frequency controlling up/down counter is operated to effect down-counting, thereby decreasing the frequency. This frequency lowering period is set to one-half of the frequency rising period from time t.sub.2 until time t.sub.3 so that exact tuning is accomplished by the decrementing of the frequency during this lowering period. The time measuring up-down counter is controlled to effect down-counting of the clock pulses and is halted at time t.sub.5 when the count value of the time measuring up/down counter becomes zero. Then a borrow output is produced and exact tuning is accomplished.
In the above-described operation, it is a common practice to attempt prevention of a chattering phenomenon so that the detection outputs are stably produced. This is typically achieved by providing a hysteresis to the level comparison characteristic of the window comparator 5. Therefore, the correlation of the productions of the detection signal S.sub.1 and S.sub.2 relative to the S-curve level are different depending upon the variation direction of the tuning frequency. As a result, however, the tuning accuracy is degraded.
Such a phenomenon will be described in far more detail with reference to the operational waveform diagram shown in FIG. 3. When the frequency is increased at a predetermined speed in an up direction as indicated by an arrow A, the time period from the falling edge of the detection signal S.sub.1 until the rising edge of the subsequent detection signal S.sub.2 is designated by T. In the case when the frequency is varied at the same predetermined speed in a down direction as indicated by an arrow B in response to the detection signal S.sub.2, the falling edge of the detection signal S.sub.2 has a time lag Ta with respect to the corresponding edge of the signal S.sub.2 in the up direction. Therefore, even if the searching operation is terminated at the time when T/2 has passed since the detection of the detection signal S.sub.2, the tuning point does not coincide with the center (f.sub.0) of the S-curve but the tuning point shifts to a higher frequency than the center frequency f.sub.0. On the other hand, in the case when after varying the frequency in the down direction as indicated by the arrow B, the frequency is then varied in the up direction upon detection of the detection signal S.sub.1, the tuning point shifts to a lower frequency than the center frequency f.sub.0. Thus, it is disadvantageous in that exact tuning cannot be accomplished. Furthermore, the above-described operation is disadvantageous in that the searching time is increased due to the fact that the searching speed is decreased immediately after the frequency enters into the S-curve frequency band. It should be noted that the frequency band of the S-curve characteristic of the FM detector is considerably wide.